Both Free Indole-3-Acetic Acid and Photosynthetic ...the growth of the plants. The present study, performed in axenic cultures, allowed the exclusion of contaminating bacteria, thus

ORIGINAL RESEARCHpublished: 16 February 2016doi: 10.3389/fpls.2016.00140

Frontiers in Plant Science | www.frontiersin.org 1 February 2016 | Volume 7 | Article 140

Edited by:

Jan Kofod Schjoerring,

University of Copenhagen, Denmark

Reviewed by:

Nicola Tomasi,

University of Udine, Italy

Tamas Adam Komives,

Plant Protection Institute, Hungary

*Correspondence:

Raquel Esteban

[email protected];

Jose F. Moran

[email protected]

†These authors have contributed

equally to this work.

Specialty section:

This article was submitted to

Plant Nutrition,

a section of the journal

Frontiers in Plant Science

Received: 28 July 2015

Accepted: 27 January 2016

Published: 16 February 2016

Citation:

Esteban R, Royo B, Urarte E,

Zamarreño ÁM, Garcia-Mina JM and

Moran JF (2016) Both Free

Indole-3-Acetic Acid and

Photosynthetic Performance are

Important Players in the Response of

Medicago truncatula to Urea and

Ammonium Nutrition Under Axenic

Conditions. Front. Plant Sci. 7:140.

doi: 10.3389/fpls.2016.00140

Both Free Indole-3-Acetic Acid andPhotosynthetic Performance areImportant Players in the Response ofMedicago truncatula to Urea andAmmonium Nutrition Under AxenicConditionsRaquel Esteban 1*†, Beatriz Royo 1 †, Estibaliz Urarte 1, Ángel M. Zamarreño 2,José M. Garcia-Mina 2 and Jose F. Moran 1*

1 Laboratory of Plant Physiology and Agrobiology, Institute of Agrobiotechnology, IdAB-CSIC-UPNA-Government of Navarre,

Public University of Navarre, Mutilva, Spain, 2Group—CMI Roullier, Department of Environmental Biology, Agricultural

Chemistry and Biology, University of Navarre, Pamplona, Spain

We aimed to identify the early stress response and plant performance of Medicago

truncatula growing in axenic medium with ammonium or urea as the sole source

of nitrogen, with respect to nitrate-based nutrition. Biomass measurements, auxin

content analyses, root system architecture (RSA) response analyses, and physiological

parameters were determined. Both ammonium and ureic nutrition severely affected the

RSA, resulting in changes in the main elongation rate, lateral root development, and

insert position from the root base. The auxin content decreased in both urea- and

ammonium-treated roots; however, only the ammonium-treated plants were affected

at the shoot level. The analysis of chlorophyll a fluorescence transients showed that

ammonium affected photosystem II, but urea did not impair photosynthetic activity.

Superoxide dismutase isoenzymes in the plastids were moderately affected by urea and

ammonium in the roots. Overall, our results showed that low N doses from different

sources had no remarkable effects onM. truncatula, with the exception of the differential

phenotypic root response. High doses of both ammonium and urea caused great

changes in plant length, auxin contents and physiological measurements. Interesting

correlations were found between the shoot auxin pool and both plant length and the

“performance index” parameter, which is obtained from measurements of the kinetics

of chlorophyll a fluorescence. Taken together, these data demonstrate that both the

indole-3-acetic acid pool and performance index are important components of the

response of M. truncatula under ammonium or urea as the sole N source.

Keywords: ammonium, auxin, OJIP curve, nitrate, root system architecture, urea

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Esteban et al. Ammonium and Urea Nutrition Effect

INTRODUCTION

Nitrate (NO−3 ), ammonium (NH+

4 ), and urea [CO(NH2)2] arethe main forms of nitrogen (N) present in agronomic fertilizersand, thus, are the principal sources of N taken up by crops.However, in agronomic soils, the scenario is much more complexbecause plants are not the only consumers of these nutrients,which can also be taken up and transformed by microorganismsin the rhizosphere. For instance, urea can be rapidly degraded bythe soil microbiota in a manner that depends on the availabilityof microbial species and environmental conditions, such astemperature (Bremmer and Krogmeier, 1988). Additionally, thepresence of nitrifying microorganisms, such asNitrosomonas andNitrobacter species, can lead to differential production of NO−

3(Hollocher, 1981; Ferguson et al., 2007). This NO−

3 productionoften contributes positively to plants, as low NO−

3 contentsin ammonium-containing media can trigger positive signalingeffects on plants and, hence, reduce NH+

4 stress (Houdusse et al.,2005; Garnica et al., 2010; Hachiya et al., 2012). Moreover, soilmicroorganisms have been demonstrated to act as hormoneproducers, especially for auxins, gibberellins, cytokinins, andabscisic acid (Reddy and Saravanan, 2013). All of these factsincrease complexity when studying N nutrition in agronomicmodels. Thus, growing plants and modeling their response inaxenic cultures is of great importance for assessing the ability ofplants to use and respond to different N sources.

Once NO−3 is assimilated, it must be reduced to NH+

4 beforeit can be incorporated into amino acids, with consequentredox/energy consumption, while NH+

4 can directly beassimilated, with a lower energy cost. Therefore, for theplant economy, NH+

4 may be thought of as a favorable Nnutrient. However, the presence of NH+

4 as the only N sourceis paradoxically toxic to most plants, and plant biomass,phenotypic changes and the plant stress response depend onthe concentration and the relative tolerance capacity of theplant species or variety (Gerendás et al., 1998; Cruz et al., 2011;Li et al., 2014). Although the mechanisms of toxicity are notfully resolved, futile cycling associated with passive diffusion ofNH+

4 across the plasma membrane, followed by NH+4 extrusion,

appeared to be responsible for an unfavorable energetic balancein plant cells (Britto and Kronzucker, 2002). Subsequent dataon the uptake of 15N-labeled NH+

4 and the natural abundanceof the isotope in plants revealed passive transport of ammoniagas through plant cell membranes, which is correlated with thesusceptibility of plants to ammonia (Ariz et al., 2011b). Thisevidence was further confirmed through flux analyses with theshort-lived radioisotope 13N (Coskun et al., 2013). Ammoniumnutrition has also been reported to modify the redox state

Abbreviations: ASC, ascorbate; IAA, indole-3-acetic acid; DHA,

dehydroascorbate; GSH, glutathione; GSSG, oxidized glutathione; hGSH,

homoglutathione; hGSSG, oxidized homoglutathione; jDo, quantum yield for

energy dissipation; jEo, quantum yield of electron transport; jPo, quantum yield

of primary photochemistry; N, nitrogen; NH+4 , ammonium; NO−

3 , nitrate; OJIP,

chlorophyll a fluorescence transients; PiAbs, performance index; yo, efficiency with

which a trapped exciton can move an electron into the electron transport chain;

RC, reaction center; ROS, reactive oxygen species; RSA, root system architecture;

SOD, superoxide dismutase.

in several species, consequently changing the homeostasis ofreactive oxygen species (ROS; Medici et al., 2004; Patterson et al.,2010). Increases in mitochondrial ROS levels in leaves have beendescribed as being associated with NH+

4 stress in some plants,such as Arabidopsis (Podgórska et al., 2013; Podgórska and Szal,2015). However, it was concluded that the ammonium-generatedstress observed in spinach and pea plants was not oxidative stress(Domínguez-Valdivia et al., 2008). Hence, the relationship ofNH+

4 with oxidative stress in plants is still an open question(Bittsánszky et al., 2015).

The possible toxicity of urea is also a matter of debate. Ingeneral, similar effects on plants to those described for NH+

4 havebeen reported for urea, but with a lower intensity (Houdusseet al., 2005). Although the understanding of NH+

4 utilization hasimproved in the last two decades (Gerendás et al., 1998; Witteet al., 2002; Mérigout et al., 2008; Zanin et al., 2014, 2015), thephysiological aspects of urea acquisition in plants grown underaxenic conditions have only been investigated in Arabidopsis andrice (Wang et al., 2013; Yang et al., 2015). Therefore, there are stillimportant gaps in our knowledge of the mechanisms of nitrogennutrition in plants under axenic conditions, including the role ofurea as an accessible nitrogen source for crops.

Here, we report a standardized method for growing the modellegumeMedicago truncatula in axenic culture using NO−

3 , NH+4 ,

or urea as the sole source of nitrogen, managing control of thepH, and growing conditions. The physiological characteristicsof this species in association with urea nutrition are unknown.Moreover, this species, which belongs to the family Leguminosae,has been described as quite tolerant to exclusive NH+

4 nutrition,although legumes also suffer from phenotypic and physiologicalchanges under different N sources (Domínguez-Valdivia et al.,2008; Ariz et al., 2011a). This tolerance could be related to(i) the ability to fix nitrogen through endosymbiosis with soilbacteria forming nodules and to (ii) the capacity to assimilate theammonium produced by the endosymbiont. During the growthof legumes with N fixed as the only N form, the endosymbiontmay produce oxidative forms of N that may have effects duringthe growth of the plants. The present study, performed inaxenic cultures, allowed the exclusion of contaminating bacteria,thus avoiding nodulation, and the procedures for the analysisof comparative N nutrition were standardized. Under theseconditions, we observed a differential phenotypic response tothe N source, with a portion of the response being associatedwith photosynthetic performance and the auxin contents of theplants.

MATERIALS AND METHODS

The Plant Growth SystemSeeds of M. truncatula Gaertn. ecotype Jemalong were scarifiedwith 95% sulfuric acid for 8min and then washed withsterile water and further surface sterilized with 50% sodiumhypochlorite for 5min, followed by washing again with sterilewater until the pH reached 7. The seeds were subsequentlygerminated on 0.4% agar (w/v) plates at 14◦C in darknessfor 72 h. The germinated seeds were transferred in a sterile






laminar flow cabinet to glass pots (five per pot) containing100ml of Fahraeus medium with 5 g l−1 of phytagel asa nutrient medium. This medium contained 0.9mM CaCl2,0.5mM MgSO4, 0.7mM KH2PO4, and 0.8mM Na2HPO4, asmacronutrients and 20µM ferric citrate, 0.8µMMnCl2, 0.6µMCuSO4,0.7µMZnCl2, 1.6µMH3BO3, and 0.5µMNa2MoO4, asmicronutrients. Nitrogen was applied as NO−

3 using Ca (NO3)2,or as NH+

4 using (NH4)2SO4 or as urea. The ammonia- andurea-fed plants were supplemented with 0.5 or 12.5mM CaSO4

to compensate for the Ca2+ supplied together with the NO−3

treatment. Two different scenarios were explicitly considered: alow N supply (1mM) and a high N concentration (25mM). Thelowest N-dose was selected following the previous results of thegroup, in order to have a non-limiting N availability (Ariz et al.,2010). The highest dose was increased to 25mM, as in agar mediahigher doses are usually necessary to have an excess of NH+

4 (Liet al., 2010). During the preparation of the axenic media, the pHwas controlled in all cases before and after autoclaving becausewe have observed that autoclaving some solutions differentiallyaffects the pH of themedia (data not shown). Thus, the CaCl2 andN sources were sterilized by filtering through cellulose acetatefilters and added to the rest of medium after autoclaving. Thisprocess allowed us to maintain a specific pH = 6.5 at the timeof the sowing of seeds (Figure 1). The plants were grown ina growth chamber for 15 days at a day/night temperature of24.5/22◦C, under 80% relative humidity, with a 16/8 h day/nightphotoperiod and 70µmoles m−2 s−1 of photosynthetically activeradiation. The pH was controlled before the experiment and afterthe growth of the plants, and it was adjusted if necessary atthe beginning of the experiment. Phosphate buffer was chosenin order to avoid the use of N-based pH buffers. An aliquotof each medium was collected under sterile conditions afterautoclaving and the pH was measured (pH day 0) using a Basic20 pH-meter (Crison). After the harvesting of the plant material,the pH was also measured (pH day 15) directly in the pots.Five replicates of each treatment and time were measured toobtain the results shown in Figure 1. After each harvest, theagar media in all pots was routinely stained with 5ml of a0.1% (w/v) solution of methyl red as a pH indicator (Figure 1).When any pot exhibited pH levels outside the range shown inFigure 1 (outside the range of the average ± S.E) was excluded.The sterility monitoring over the 15 days was performed bycarefully controlling the agar medium, when no microorganismgrown was appreciated the medium was used for harvestingand further analysis. Harvesting was always conducted at 6 hafter the onset of the light period. A pool of five plants fromdifferent pots was collected (shoots and roots), weighed, and ovendried or frozen in liquid N, and stored at −80◦C for furtheranalyses.

Growth Determination: Total Biomass, DryWeight, Plant Length, and Root SystemArchitectureDuring harvesting, 15 plants from each treatment were randomlyselected to determine the fresh weight of both the shoots androots. The dry weight was determined after incubation in a dry

FIGURE 1 | Effects of NO−

3, NH+

4, and urea on pH. pH on days 0 and 15 of

the experiment under the different treatments: low doses of NO−

3 , NH+

4 , and

urea (closed circle, square, and triangle, respectively) and high doses of NO−

3 ,

NH+

4 , and urea (opened circle, square, and triangle, respectively). The values

are the mean ± S.E (n = 15). Photographs of the pH indicator methyl red on

day 15 of the experiment for each of the treatments are shown (NL, AL, UL:

low doses of NO−

3 , NH+

4 , and urea, respectively, and HH, AH, UH: high doses

of NO−

3 , NH+

4 , and urea, respectively).

oven at 80◦ C for 2 days. The shoot length was measured inthese plants as the distance from the shoot base to the apicalmeristem. The quantification of root growth and architecturewas performed with the semi-automated image analysis softwareSmartRoot (Lobet et al., 2011) by estimating growth rates,the accumulated length of lateral roots and the position ofinsertion from the root base. For this, five M. truncatulaplants were grown as previously described on agar plates,and their root system was photographed every 4 days during15 days (2D photographs). A dataset containing architecturaldescriptions of the root system under the different N sources wasestablished.

Auxin ContentThe concentration of indole-3-acetic acid (IAA) was analyzedin shoot and root extracts using high performance liquidchromatography-electrospray-mass spectrometry (HPLC-ESI-MS/MS) as described in Jauregui et al. (2015). The extraction andpurification of this hormone was carried out using the followingmethod also described in Jauregui et al. (2015). Hormones werequantified by HPLC-ESI-MS/MS using an HPLC (2795 AllianceHT; Waters Co., Milford, MA, USA) coupled to a 3200 Q-TRAPLC/MS/MS System (Applied Biosystems/MDS Sciex, Ontario,Canada), equipped with an electrospray interface. A reverse-phase column (Synergy 4mm Hydro-RP 80A, 150 × 2mm;Phenomenex) was used. The detection and quantification of thehormone was carried out using multiple reactions monitoring inthe negative-ion mode, employing multilevel calibration curveswith the internal standards as described in Jauregui et al.(2015).






Chlorophyll a Fluorescence Rise KineticsMeasurementsMeasurements of chlorophyll a fast fluorescence transients(OJIP) were performed in M. truncatula leaves by a FluorPenFP 100 (Photon System Instrument, Brno, Czech Republic). Thistechnique allows an estimation of photosynthetic performanceand denotes the flow of energy through photosystem II, whichis a highly sensitive signature of photosynthesis (Strasser et al.,2000). For a more detailed review, see Strasser et al. (2000,2004) and Stirbet and Govindjee (2011). Prior to measurements,leaves were dark-adapted for a night period (14 h), to allow thecomplete relaxation or oxidation of reaction centers in order todetermine Fo. For excitation, bandpass filters of light emittingdiodes with 697–750 nm provided of 3000µmol photons m−2

s−1 at leaf sample and fluorescence transients were induced andrecorded during 2 s at a frequency of 10µs, 100µs, 1ms, and10ms for the time intervals of 10–600µs, 0.6–14ms, 14–100ms,and 0.1–2 s, respectively. The fluorescence values at 40µs (Fo,step 0, all reaction centers of the photosystem II are open),100µs (F100), 300µs (F300), 2ms (step J), 30ms (step I), andmaximal (FM , step P, closure of all reaction centers) were takeninto consideration. Cardinal points of the OJIP curve and derivedparameters were calculated with the Fluorpen 2.0 software, basedon the theory of energy fluxes in biomembranes by the formulasderived from Strasser et al. (2000, 2004). In this paper, we haveconsidered fluorescence parameters derived from the extracteddata and (i) normalized signals as Vj (Fj–Fo/FM–Fo) and Vi (Fi–Fo/FM–Fo), (ii) quantum yields and efficiencies, (iii) the specificfluxes per active reaction center (RC) for absorption (ABS/RC),trapping (TR0/RC), electron transport (ET0/RC), and dissipation(DI0/RC). We have also analyzed the performance index, PiAbs,which is the potential performance index for energy conservationfrom photons absorbed by photosystem II to the reduction ofintersystem electron acceptors. In this paper, we do not analyzethe events relative to PSI (Zubek et al., 2009). The formulas usedto calculate the above parameters and detailed information aregiven in the Supplementary Table S1.

Superoxide Dismutase Activity (SOD)In gel SOD activity of roots and shoots of M. truncatula wasmeasured as described by Beauchamp and Fridovich (1971),and Asensio et al. (2011, 2012). SOD isoforms identificationwere made according to known mobility of SOD on nativegels and based on the differential inhibition of SOD activityon gels pre-incubated with either 3mM KCN, which inhibitedthe CuZnSODs, or 5mM H2O2 for 1 h, which inhibited FeSOD(Asensio et al., 2011, 2012). The in-gel SOD activity assays wereperformed at least three times in order to ensure the consistencyof the results.

Metabolite AnalysesThe reduced and oxidized forms of the antioxidant metabolites(ascorbate, glutathione, and homoglutathione) were determinedin shoots and roots as described in Zabalza et al. (2007). Inbrief, frozen samples (0.2 g) were ground in a mortar withliquid nitrogen and then homogenized with 2ml extractionbuffer (2% metaphosphoric acid; 1mM EDTA). The homogenate

was centrifuged at 4400 g for 2min at 4◦C and filtered.Ascorbate (ASC), glutathione (GSH), and homoglutathione(hGSH) contents were analyzed by high-performance capillaryelectrophoresis in a Beckman Coulter P/ACE system 5500(Fullerton, CA) using capillary tubing (50mm; 30/37 cm long).ASC, GSH, and hGSH were detected with a diode arraydetector by setting wavelength at 265 for ASC, and 200 nmfor reduced thiol tripeptides. To determine dehydroascorbate(DHA), oxidized glutathione (GSSG), and oxidized hGSH(hGSSG) the extracts were reduced with DTT as described byDavey et al. (2003), then total ASC, GSH, and hGSH wereanalyzed by HPCE using standards as reference. DHA, GSSG,and hGSSG contents were determined as the difference betweenthe total antioxidant pools and levels of their reduced forms.Redox status was determined as the ratio between the reducedform (ASC, GSH, or hGSH) and the total antioxidant content(oxidized + reduced). In this paper, we have considered thepool of GSH and hGSH together due to they are homologousmolecules regarding the plant redox state. Protein contents werequantified using a dye-binding Bradford micro-assay (Bio-Rad,Watford, UK) with bovine serum albumin as a standard. Atharvest, the relative content of chlorophyll in leaves (SPAD index)was determined using the chlorophyll meter SPAD-502 (Minolta,Japan). This instrument provides a relative measurement ofchlorophyll in leaves through the evaluation of the changes of thetransmittance in the 600–700 nm region of the visible spectra andin the near infrared region of the spectra.

StatisticsDifferences among treatments were evaluated with one-wayANOVA and post-hoc Student-Newman-Keuls test. All datawere tested for normality (Kolmogorov–Smirnof test) andhomogeneity of variances (Cochran test) and log-transformed ifnecessary. When this failed to meet ANOVA assumptions, theywere analyzed using the nonparametric Mann–Whitney test. Theresulting p-values were considered to be statistically significantat α = 0.05. Linear regressions were used to analyze relationshipin Figure 7. Calculated p-values, coefficients and regression linesare indicated whenever significant at α= 0.05. Statistical analyseswere performed with IBM SPSS Statistics for Windows, Version21.0. Armonk, NY: IBM Corp.

RESULTS

The Plant Growth SystemPrevious experiments performed by our group revealed thatMurashige and Skoog medium is incompatible with agarsolidification under a high NH+

4 concentration (data notshown). Therefore, an original differentially N-supplementedaxenic medium was used to grow M. truncatula plants underaxenic conditions with NO−

3 , NH+4 , or urea for 15 days. The

growth medium, containing 1.5mM phosphate buffer, exhibiteda significant buffering capacity, similar to that of Murashige andSkoog medium, which contains 1.25mM phosphate. Hence, thepH of the growth medium during the experiment changed littlefor the plants grown using NO−

3 . However, in the plants grownwith NH+

4 as the sole source of N, there was a significant decrease






in pH, with pH measurements of 5.2 and 4.5 being observedfor the 1mM and 25mM ammonium treatments, respectively.Intermediate decreases were found in association with ureanutrition at both doses (Figure 1).

Differential Root System Architecture andGrowth of Seedlings Under the Tested NSourcesTreatment with either NH+

4 or urea at a low dose causeda significant reduction of the fresh weight of the shootsof M. truncatula seedlings compared with NO−

3 treatment(Figure 2A). At a high dose, only the plants fed with ureashowed reduced shoot growth, which was related to thedifferential root/shoot biomass ratio, with greater investment inthe roots being observed under this type of nutrition (Figure 2C).Interestingly, NH+

4 had a negative effect on fresh weight at a lowdose, but not at high dose, where a different root/shoot ratio mayinfluence changes in fresh weight. Root growth was only affectedunder NH+

4 nutrition at a low N concentration. The differenceswere not significant for dry matter in either the roots or theshoots (Figure 2B). In terms of the total biomass (Figure 2B),both low and high doses of NH+

4 decreased the growth perplant, which was also reduced under high-dose urea-fed plants.Significant changes in the architecture of the roots occurredduring development (Figure 3 and Supplementary Table S2).We confirmed that there was a significant change in the mainroot elongation rate in each of the treatments, with a delay

in growth being observed during the first 4 days in plantsgrown under a high dose of NH+

4 (Figure 3A). Interestingly,in the plants fed with urea at a low dose, enhancement of theelongation of the main root was observed after the first 4 daysof growth. All treatments reduced the main root elongationrate after the 8th day of growth. The differences in lateral rootgrowth and elongation were also evident and significant at theend of the experiment. In both the NH+

4 - and urea-fed plants,the accumulated lateral root growth was lower than in the NO−

3 -fed plants (Figures 3B,C). This differential accumulated lateralgrowth in the low-dose treatments was due to a greater numberof lateral roots growing more slowly in NH+

4 -treated roots and tothe presence of fewer lateral roots in urea-fed roots (Figure 3D).No significant differences in the number of lateral roots werefound for the high-dose treatments (Figure 3E). Moreover, aremarkable relationship was observed regarding the length of thelateral roots and the position of insertion from the root base, withopposite patterns being recorded in the NH+

4 - and NO−3 -grown

plants, as illustrated in Figures 3I,J. No significant relationshipwas detected in the urea-fed plants, probably due to the lownumber of lateral roots (data not shown; p = 0.075). Thesedifferences in plant growth in relation to the type of nutritionmight reflect disparities in N uptake and/or assimilation, butthey may also be related to differential hormone signaling.Accordingly, the free IAA content was analyzed. The NH+

4 (lowand high doses)-fed plants showed a decrease in the IAA contentin the roots compared with the NO−

3 -grown plants (Figure 4).The decrease in the IAA content was even more pronounced


3, NH+

4, and urea on biomass. Distribution of the plant biomass of roots and shoots (A), total biomass (B), and the root/shoot ratio (C)

expressed on the basis of the fresh weight (colored bars) and dry weight (white bars) (mg) per plant subjected to the different treatments (low and high doses of NO−

3 ,

NH+

4 , and urea). The values are the mean ± S.E (n = 15). Different letters (capital letters for fresh weight and lowercase letters for dry weight) denote statistically

significant differences at α = 0.05 using the Student–Newman–Keuls test. To standardize the variances, one data point was replaced by the mean of the group for the

root fresh weight, and 1 degree of freedom was subtracted from the residual (Winer et al., 1991).







3, NH+

4, and urea on the root system architecture. Main root elongation rate (A) and accumulated (

∑) lateral root length under a low

dose (B) and high dose (C). Inset panels show the changes in the number of lateral roots under each treatment for a low dose (D) and high dose (E). The values are

the mean of 5–3 ± S.E. An analysis of variance (ANOVA) was performed in (A) considering the treatment as a fixed factor. Different superscripted letters denote

statistically significant differences at α = 0.05 using the Student-Newman-Keuls test. An absence of letters indicates that there were no significant differences. For the

nitrate (F), ammonium (G) and urea treatments at a low dose, a representative image from each condition at day 14 is shown (H) Relationship of the insertion position

from the root base with the length of lateral roots for a low dose of NO−

3 (I) and a low dose of NH+

4 (J). More information regarding the phenotypic analysis is provided

in Supplementary Table S2.

and significant when analyzed based on dry weight (data notshown). However, the most remarkable decrease was observedin the shoots under 25mM NH+

4 . In contrast, urea did not alterIAA contents at the shoot level but did provoke a decrease inIAA at the root level in the low-dose treatment compared withNO−

3 treatment. Indeed, the decrease was remarkably higher inthe presence of urea at a low dose than was observed for NH+

4 .

Stress IndicatorsPhotosystem II is considered to play a key role in the responseof photosynthesis to environmental stress (as may be the casefor NH+

4 or ureic nutrition), and OJIP monitoring is a methodfor investigating the events occurring in photosystem II. Thistechnique allows in vivo evaluation of plant performance in termsof biophysical parameters quantifying photosynthetic energyconservation (Strasser et al., 2000, 2004). The induction ofrelative variable fluorescence (Figure 5) and the parametersderived from this curve (Table 1) showed no remarkabledifferences between the treatments at low doses. However, therewas a clear effect of a high dose of N nutrition, as indicated by

the relative amplitude of the J–I phase (1–10ms) plotted on alogarithmic time scale for NH+

4 compared with the NO−3 and

urea treatments (Figure 5). Much more pronounced differencesbetween treatments were revealed by the parameters derived afterfurther analyses of the curves of fluorescence according to theOJIP test (see the Material and methods and SupplementaryTable S1). Indeed, the relative variable Chl fluorescence at 2ms(Vj) was significantly lower under high NH+

4 doses. Curiously,the maximum quantum yield of the primary photochemistry(ϕPo) and the quantum yield for energy dissipation (ϕDo) weresignificantly higher under urea at a high dose, while the efficiencywith which a trapped exciton could move an electron into theelectron transport chain (9o) and the quantum yield of electrontransport (ϕEo) were significantly lower under NH+

4 at a highdose. Regarding the specific changes per reaction center (RC),only the electron transport flux per RC (ETo/RC) was affected inhigh-dose NH+

4 -fed plants. The parameter PiAbs, which combines(i) the RC density expressed on an absorption basis, (ii) thequantum yield of the primary photochemistry, and (iii) the abilityto feed electrons into the electron chain between photosystem







3, NH+

4, and urea on the IAA concentration.

High and a low doses of NO−

3 , NH+

4 , and urea altered the IAA concentration

(pmoles g−1 fresh weight) in M. truncatula seedlings. The values are the mean

± S.E (n = 3). Different capital letters denote statistically significant differences

at α = 0.05 after the Student–Newman–Keuls test.

II and I (Cascio et al., 2010), exhibited a significantly highervalue under a high dose of urea, which was even higher than thevalue observed under NO−

3 . Conversely, PiAbs showed the lowestvalues under NH+

4 at both doses (presenting a lower value inthe high-dose treatment; Table 1). Moreover, slight differencesin chlorophylls content (SPAD units) were observed, with asignificant decrease being recorded in plants fed with 25mMNH+

4 . No differences in the total protein content were detected(Table 2).

Regarding antioxidants, there were no significant changes inthe size and general oxidation state of the antioxidant pools inthe roots. On the contrary, the NH+

4 - and urea-supplied plantspresented a lower reduced pool of GSH+hGSH in the shoots.On the other hand, there were detectable differences in theASC oxidation pool in the shoots, with higher contents beingrecorded in urea-grown seedlings (Table 2). Regarding SODactivity (Figure 6), four isozymes were found in both the leavesand roots (MnSOD, FeSOD, cytosolic-CuZnSOD, and plastidial-CuZnSOD). However, the response was different betweenthe two tissues. In the roots (Figure 6A), no or very slightdifferences were observed for the isoenzyme MnSOD, indicating

that a special level of protection for mitochondria-associatedsuperoxide production was not required under any of the testedconditions. On the other hand, cyt-CuZnSOD was somewhataffected by the type of nutrition, increasing under low and highurea, but showing no significant ncreases under NH+

4 treatment.Increases (although not significant) could also be observed forFeSOD in plants grown under high N doses (NO−

3 and NH+4 ).

Unlike FeSODs from determinate nodule-forming legume plants,such as soybean, cowpea, or Lotus japonicum, where FeSODmembers occur in the plastids or cytosol, the FeSODs of theMedicago family are plastidial proteins (Moran et al., 2003;Asensio et al., 2012). Additionally, plastid-associated CuZnSODwas significantly induced in plants fed with high NH+

4 and lowor high urea. On the whole, it appears that plastidial enzymesare more affected, and ureic nutrition appears to impact a greaternumber of SOD isoenzymes. Conversely, no important changeswere observed in the shoots (Figure 6B), with the exception of adecrease of cyt-CuZnSOD activity under high NO−

3 treatment.Overall, these results suggest that small adjustments in theantioxidant pool may prevent major perturbations in redox ratiosunder differential N nutrition.

Our study also demonstrated interesting relationships amongthe studied parameters (Figure 7). PiAbs (a parameter obtainedfrom the induction of chlorophyll a) showed a significant positiverelationship with the shoot IAA content (r2 = 0.653; p <

0.05; Figure 7C). Moreover, the shoot IAA content exhibited asignificant positive relationship with the total length of the plant(r2 = 0.815; p < 0.05), as did the total IAA content with thetotal length of the plant (data not shown; r2 = 0.778; p < 0.05).There was also a tendency for a greater length to be correlatedwith a higher performance index, although not significantly (r2

= 0.617; Figure 7B). Interestingly, total biomass on a fresh basiswas not correlated with the IAA content (Figure 7D), nor was theroot IAA content correlated with root length (data not shown).Taken together, these findings suggested that NO−

3 - and urea-fed seedlings showed higher performance indices than NH+

4 -fedplants, which was clearly correlated with higher IAA contents,potentially resulting in better length growth for these plants.

DISCUSSION

The Importance of pH ControlGrowing crop plants using NH+

4 or urea as the only N sourceunder axenic conditions is a poorly examined method. Fewstudies are performed under these conditions, especially inplants grown with urea (Gerendás et al., 1998; Wang et al.,2013; Yang et al., 2015), and it is therefore a challengingtask. Appropriate control of the pH in the medium after thesterilization process is important to achieve the correct growthof plants and appropriate evaluation of urea or NH+

4 nutritiontreatments because the autoclaving process may change the pHof the solution when certain nutrients are autoclaved together(see the Materials and Methods). Moreover, it is widely knownthat the pH of the medium may have an effect on nutrientassimilation during the development of seedlings. Our systemstandardized the control of pH during the preparation of the







3, NH+

4, and urea on fast chlorophyll a fluorescence transients (OJIP) from dark-adapted leaves of M. truncatula plants

grown under the different treatments at low (A) and high doses (B). Each transient is plotted on a logarithmic time scale from 40µs and 1 s and expressed as

the relative variable fluorescence (Vt) after double normalization of Fo and Fm. The values are the means (n = 3–4).

TABLE 1 | Numerical values for fluorescence parameters derived from chlorophyll a fast florescence transients in the leaves of M. truncatula: (i)

normalized data as Vj and Vi, (ii) quantum yields and flux ratios as ϕPo, 9o, ϕEo, (iii) performance index (Pi_Abs), and (iv) specific energy fluxes per Q−

Areducing photosystem II centers as ABS/RC, TR0/RC, ET0/RC, and DI0/RC.

Parameter Low N supply High N supply

Nitrate Ammonium Urea Nitrate Ammonium Urea

Normalized data Vj 0.42±0.02a 0.45± 0.01a 0.44±0.02a 0.45± 0.03a 0.56± 0.04b 0.42± 0.02a

Vi 0.78±0.03 0.78± 0.02 0.79±0.01 0.79± 0.01 0.78± 0.02 0.74± 0.00

Quantum yields and flux ratios ϕPo 0.74±0.00a 0.75± 0.00ab 0.76±0.01ab 0.76± 0.00ab 0.74± 0.00a 0.77± 0.00b

9o 0.58±0.02a 0.55± 0.01a 0.56±0.02a 0.55± 0.03a 0.44± 0.04b 0.58± 0.02a

ϕEo 0.43±0.01a 0.42± 0.00a 0.43±0.02a 0.42± 0.02a 0.33± 0.03b 0.44± 0.02a

ϕDo 0.26±0.00a 0.25± 0.00ab 0.24±0.01ab 0.24± 0.00ab 0.26± 0.00a 0.23± 0.00b

Pi_Abs 1.12±00.07ab 1.06± 0.04ab 1.22±0.15ab 1.19± 0.14ab 0.67± 0.12a 1.39± 0.16b

Specific energy fluxes ABS/RC 3.58±0.04 3.54± 0.12 3.39±0.08 3.32± 0.03 3.54± 0.17 3.29± 0.07

TRo/RC 2.67±0.04 2.66± 0.07 2.57±0.04 2.53± 0.08 2.63± 0.11 2.52± 0.04

ETo/RC 1.54±0.03a 1.47± 0.06a 1.44±0.03a 1.39± 0.0a 1.17± 0.14b 1.46± 0.04a

DIo/RC 0.92±0.01 0.88± 0.05 0.82±0.04 0.79± 0.01 0.92± 0.06 0.77± 0.03

Definitions and formulae are given in the Materials and Methods and in Supplementry Table S1. The values are the means ± S.E. from independent measurements (N = 3–4). Ananalysis of variance (ANOVA) was performed, considering the treatments as fixed factors. Different superscripted letters denote statistically significant differences at α = 0.05 using theStudent-Newman-Keuls test. An absence of letters indicates that there were no significant differences.

media and the development of the seedlings using the routine

color indicator methyl red (Figure 1). Our results showed that

the pH did not decrease greatly under all tested conditions,

except at a very high dose of NH+4 , where the pH could drop

to 4.5. Under this condition, the plants showed a maximum

quantum yield of primary photochemistry similar to the rest of

the treatments (ϕPo; Table 1). In contrast, in non-buffered full-

strength solutions containing NH+4 , pH values lower than 4.0 can

occur, which renders the solution unsuitable for healthy growth(Zaccheo et al., 2006).

How are the Shoots and Roots Affected byNH+

4 and Urea Nutrition?One of the most dramatic plant adaptations for ensuringadequate N acquisition is the modulation of the RSA (Fordeand Lorenzo, 2001; Hodge, 2004; Smith and De Smet, 2012;Forde, 2014). A detailed description of root development underNH+

4 nutrition has not yet been provided, and only a fewstudies have described the changes in the RSA under ureanutrition (Yang et al., 2015; Zanin et al., 2015). Our resultsobtained in both NH+

4 - and urea-fed plants at both tested doses






TABLE 2 | Chlorophylls by SPAD units, ascorbate (ASC), and glutathione+homoglutathione (GSH+ hGSH) redox status and protein content in the tissues

(shoots and roots) of M. truncatula plants grown using NO−

3, NH+

4, and urea media as the only N source.

Tissue n Low High

Nitrate Ammonium Urea Nitrate Ammonium Urea

Chlorophylls (SPAD units) Leaves 14 29.1± 1.2abc 25.9±1.5ab 26.3± 1.6ab 30.8±1.0c 25.4±1.1a 29.7± 1.6bc

ASC/(DHA+ASC) Shoot 4 0.48± 0.02a 0.53±0.03ab 0.57± 0.04b 0.62±0.02b 0.53±0.00ab 0.58± 0.02b

Root 4 0.60± 0.00a 0.57±0.0ab 0.57± 0.02ab 0.54±0.02b 0.55±0.01ab 0.56± 0.01ab

(GSH+hGSH)/(GSSG+hGSSG+GSH+

hGSH)

Shoot 4 1.37± 014a 1.29±0.21a 1.00± 0.01ab 1.39±0.25a 0.77±0.25ab 0.43± 0.02b

Root 4 0.76± 0.14a 0.95±0.21a 0.58± 0.06a 0.73±0.07a 0.83±0.28a 0.54± 0.06a

Protein content (mg/g−1 dry weight) Shoot 8 10.4± 2.0 9.6±1.6 8.2± 1.7 12.5±2.0 12.5±1.8 11.4± 1.8

Root 5 8.6± 1.3 8.4±1.1 8.1± 1.8 11.1±1.3 10.1±1.1 8.7± 1.1

The values represent the mean± S.E. (n= 4–14). Different superscripted letters denote statistically significant differences at α = 0.05 using the Student-Newman-Keuls test. An absenceof letters indicates that there were no significantly differences.

FIGURE 6 | Effect of NO−

3, NH+

4, and urea on superoxide dismutase activity in the roots (A) and leaves (B) of 2-weeks old Medicago truncatula plants

grown under axenic conditions with NO−

3, NH+

4, or urea as the exclusive source of N. The gels are representative examples of three replicate experiments.

The densitometry results for the isoenzyme bands are expressed in the lower panel as percentages relative to the corresponding control reference bands (1mM NO−

3 ).

All lanes of the 15% native-PAGE gel were loaded with 30µg of protein. The values are the means ± S.E. (n = 3). Different letters denote statistically significant

differences at α = 0.05 using the Student–Newman–Keuls test. To standardize the variances, one data point was replaced with the mean for the group for shoot

cyt-CuZnSOD and root pl-CuZnSOD, and consequently, 1 degree of freedom was subtracted from the residual in each case (Winer et al., 1991). An absence of letters

indicates that there were no significant differences.

(Figures 1A, 3A–C) agree with the phenotype described underNH+

4 stress in the majority of plants (Britto and Kronzucker,2002). These changes imply the existence of shortened roots, witha significant reduction of the primary root, which is explainedby the inhibition of cell elongation (Li et al., 2010) and thesuppression of lateral root elongation (Li et al., 2010, 2011;Rogato et al., 2010). Inhibition of lateral root length was alsoobserved under a high NO−

3 supply (Figure 3C; Walch-Liu et al.,2006). Interestingly, we found a negative correlation for NO−

3 -grown roots (Figure 3I) and a positive relationship for NH+

4 -grown roots (Figure 3J) when relating the position of insertionfrom the base of the lateral roots to their lengths. The absence ofa relationship in urea-fed plants was probably due to the smallnumber of lateral roots in these plants (Figure 3B), as reportedfor urea-grownArabidopsis plants (Yang et al., 2015). Conversely,Zanin et al. (2015) described a significant increase in the totaldensity and area of urea-fed maize roots.

Our results did not show a decrease in dry weight in anyof the treatments, probably due to the low biomass originatingfrom axenic cultures. Other species described as tolerant to NH+

4nutrition, such as M. truncatula, have not been found to showany decrease in dry matter contents when grown with NH+

4 asthe only N source (Domínguez-Valdivia et al., 2008; Cruz et al.,2011). Although there were no significant changes in biomass,the root/shoot ratio was significantly altered under different Nsources, as previously demonstrated for high and low N doses(Ariz et al., 2011a). The urea-treated plants showed significantgrowth reduction on a fresh weight basis, compared with theplants grown under NO−

3 nutrition, but to a lesser extent thanthe NH+

4 -grown seedlings, as described for a number of plantspecies (Houdusse et al., 2005; Mérigout et al., 2008; Garnicaet al., 2010; Yang et al., 2015). Indeed, various authors havereported that plants fed with urea suffer from N deficiency dueto an extremely low urea uptake rate (Arkoun et al., 2012; Wang






FIGURE 7 | Relationships under the effects of NO−

3, NH+

4, and urea. Relationship of the total plant length with the shoot IAA content (A) and performance index

(B). Relationship of the shoot IAA content with the performance index (C) and with shoot fresh weight (D). Points shown are the means (n = 4) ± S.E. for the plants.

et al., 2013). However, the recent description of efficient ureatransporters (Kojima et al., 2007; Wang et al., 2008; Zanin et al.,2014; Liu et al., 2015) confirms the possibility that plants mayuse urea the sole nitrogen source, with the compound beingassimilated and translocated by plants (Mérigout et al., 2008).The unchanged total biomass of urea- and NO−

3 -fed plants(Figure 2B) showed that urea is taken up by the roots withouthydrolysis in our experimental system, as the axenic conditionsprevented any microbial-based conversion of the supplied Nforms during the course of the experiments. Furthermore, nourea degradation was found to occur in the nutrient media, asno NH+

4 was detected through ion chromatography in samplesof the agar media collected randomly at the end of plantgrowth (data not shown). Finally, the similar protein contentsof the root tissues after exposure to the different N sourcesindicated efficient assimilation of urea and NH+

4 in our plants(Table 2). Accordingly, Cao et al. (2010) reported that urea-grown plants were subject to a more inefficient N distributionthan to inefficient urea uptake, and it was subsequently reportedthat plants are able to use urea (Zanin et al., 2015).

How is the IAA Pool Affected by NH+

4 andUrea Nutrition?Phenotypic changes, beyond simple morphologicaltransformation, may imply more profound regulation regardinghormone-related signals (Guo et al., 2009; Bartoli et al., 2013).Accordingly, auxin levels have been reported to regulate theelongation of the main and lateral roots during development(Péret et al., 2009; Li et al., 2011). In the present study, wedid not observe inhibition of IAA levels in the roots under a

high NO−3 supply (Figure 4), as has been described in other

studies (Tian et al., 2008; Tamura et al., 2010), which may likelyexplain why the primary length was not inhibited under a highNO−

3 supply in our experiments (Figure 3A). Conversely, inNH+

4 -fed-plants, a suppression of root IAA contents has beendescribed (Kudoyarova et al., 1997; Li et al., 2011), as observedin the present work (Figure 3C). In urea-supplied plants,transcriptomic data have also shown a marked phytohormonalimbalance, although auxin-regulated gene categories werenot affected (Yang et al., 2015). This may be explained by theabsence of NO−

3 , in urea- and NH+4 -grown seedlings. Nitrate is

essential for the uptake of IAA into root cells and therefore forauxin signaling (Krouk et al., 2010). Thus, in treatments witha complete absence of NO−

3 , the length of lateral roots may bealtered due to the absence of signaling related to the NO−

3 -auxininterplay in the roots (Krouk et al., 2010). On the other hand, thecorrelation found between total length and the shoot IAA poolinM. truncatula (Figure 7A) suggests that exogenous IAA couldcomplement the NH+

4 and urea phenotype. Accordingly, IAAwas added to the growth medium at 20µM. However, it did notpromote root development (data not shown). Barth et al. (2010)and Yang et al. (2015) also concluded that plants grown withexogenous IAA did not show any symptoms of growth recoveryfrom NH+

4 toxicity or urea nutrition, respectively. The shootsof NO−

3 -fed plants presented higher IAA levels than those ofurea-fed plants (at high doses), in contrast to what was observedby Mercier et al. (1997), who reported that urea-treated plantspresented higher IAA levels than those grown with NO−

3 . Nitratemay be not as important in the shoots as in the roots, as weobserved that the IAA contents were positively correlated with






plant length and the performance index, indicating differences inregulation and effects for both the roots and shoots. Taking intoaccount that the suppression of auxin signaling is considered toenhance stress tolerance (biotic and abiotic) (Park et al., 2007),the decline of IAA contents observed under urea and NH+

4nutrition (more pronounced) suggests that both treatments maymodify auxin contents, suggesting a trade-off mechanism forenhancing tolerance toward these conditions.

Do Urea and NH+

4 Nutrition Induce Stressin M. truncatula?Although the toxic effect of NH+

4 at the whole-plant level iswidely known (Britto and Kronzucker, 2002; Bittsánszky et al.,2015), the toxic effect of urea is still a matter of debate. Theeffects of NH+

4 on the photosynthetic machinery have beendescribed in several species, such as spinach (Lasa et al., 2002),maize (Foyer et al., 1994), and Synechocystis (Drath et al.,2008). The similar maximum quantum yield of the primaryphotochemistry observed in all of the treatments (ϕPo) revealedthat the photosynthetic apparatus of M. truncatula was notphotoinhibited under different types of nutrition, as shownin other studies (Podgórska et al., 2013; Bittsánszky et al.,2015). However, a detailed analysis of the kinetic transients ofchlorophyll a fluorescence indicated that NH+

4 at a high dosewas associated as the most sensitive phenotype, due to thelower value of PiAbs (Figure 5 and Table 1). Paradoxically, urea,which has been reported to share similar assimilation pathwaysin Arabidopsis (Mérigout et al., 2008), had the opposite effect.Not only did urea-treated M. truncatula plants maintain theirphotosynthetic capacity, but the biophysical parameters of thephotosynthetic apparatus also indicated that urea-grown plantsdisplayed a dose-dependent improvement in energy conservationfrom absorbed photons to reduction, which was even better thanthat observed in NO−

3 -grown plants. Urea applied at the leaflevel has been reported to increase leaf CO2 assimilation in N-deficient plants (Del Amor and Cuadra-Crespo, 2011). However,transcriptomic data indicated down-regulation of genes involvedin photosynthesis (Yang et al., 2015). The investigation ofwhether oxidative stress is involved in NH+

4 tolerance/toxicityhas led to contradictory conclusions. Ammonium does not giverise to oxidative stress in pea and spinach plants (Domínguez-Valdivia et al., 2008). However, it does generate stress in aquaticplants (Nimptsch and Pflugmacher, 2007), tobacco (Skopelitiset al., 2006), and Arabidopsis (Patterson et al., 2010). Most plantsexhibit more oxidized states for both antioxidants when NH+

4 issupplied (Podgórska and Szal, 2015). However, we did not findclear differences in the redox status of ASC and GSH+hGSG(Table 2), and mitochondria-associated superoxide productionwas not specifically increased under any of the conditions, incontrast to the findings of other studies conducted in A. thaliana,in which mitochondrial Mn-dependent SOD has been shown tobe particularly upregulated when NH+

4 is supplied (Podgórskaet al., 2013; Podgórska and Szal, 2015). Our results suggestedthat plastidial enzymes were more affected by both NH+

4 andurea, while urea nutrition impacted the greatest number of SODisoenzymes, which suggests that production of ROS occurs in thissituation. This observation is consistent with a transcriptomicanalysis performed in urea-fed Arabidopsis plants, in which

FeSOD genes were found to be overexpressed (Mérigout et al.,2008). In our model, the legumeM. truncatula did not appear tosuffer fromNH+

4 toxicity, as the leaves did not show any chloroticsymptoms, which are the main markers of NH+

4 toxicity (Brittoand Kronzucker, 2002). This finding agrees with the fact thatlegume plants are relatively tolerant to NH+

4 , as demonstrated bythe growth ratio between NO−

3 and NH+4 conditions (Ariz et al.,

2011a). Overall, NH+4 treatment leads to “mild” perturbation

under this system, with differential effects on the photosyntheticmachinery.

CONCLUSION

Taken together, our results showed that low N doses fromdifferent sources had no remarkable effects on M. truncatulaplant performance. The exception was the modification ofRSA, resulting in changes in both lateral root developmentand the relationship of the insertion position from the rootbase, which were clearly affected by both NH+

4 and ureatreatments and demonstrated great plasticity and modulationof M. truncatula roots. Conversely, our system revealed thatM. truncatula grown under NH+

4 nutrition at a high dosepresented a lower performance index and enhanced activity ofSOD; however, we did not find clear symptoms of a reductionin the plant antioxidant status. In plants grown with urea ata high dose, we observed SOD up-regulation, but no effecton the photosynthetic apparatus was detected. Therefore, inboth plant treatments, these reactive species may be moreinvolved in the activation of the defense system, rather than theinduction of cellular damage. Legumes have been demonstratedto be more tolerant to NH+

4 than other crops (Ariz et al.,2011a) and are probably also more tolerant to urea. Our resultsindicate that rather than being characterized by a physiologicaldisorder, urea-fed plants experienced a modest alteration of somecellular parameters (redox state and growth). Taken together,our findings indicate that NH+

4 at high dose, but not urea,represents a more stressful condition for M. truncatula in termsof biophysical parameters, with modification of the IAA poolbeing observed at the shoot level. Our results indicate that boththe IAA pool and performance index are important players inthe response of the plants to NH+

4 or urea as the sole sourceof nitrogen.

AUTHOR CONTRIBUTIONS

RE and BR conceived and performed experiments, interpreteddata, and contributed to the drafting of the manuscript. REalso supervised the whole project and wrote the manuscript.EU performed experiments and contributed to the correctionof the manuscript. AZ and JG performed IAA analyses andcontributed to the drafting of the manuscript and JFM conceivedand supervised the whole project and gave experimental adviceand contributed to the manuscript.

FUNDING

This work was supported by the grants AGL2010-16167 andAGL2014-52396-from the Spanish Ministry of Economy and






Competitiviness-Mineco. BR is a holder of a PhD fellowship fromthe Mineco. RE received a JAE-Doc-2011-046 contract from theSpanish Research Council (CSIC) of the programme≪Junta parala Ampliación de Estudios≫ co-financed by the European SocialFund.

ACKNOWLEDGMENTS

We thank Drs. Inmaculada Farran and Jon Veramendifor providing the FluorPen FP 100, and Gustavo Garijo

for technical assistance. We are greatly acknowledged toPedro M. Aparicio Tejo for advice in the drafting of themanuscript. Comments from the reviewers improved a previousversion.

SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be foundonline at: http://journal.frontiersin.org/article/10.3389/fpls.2016.00140

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Conflict of Interest Statement: The authors declare that the research was

conducted in the absence of any commercial or financial relationships that could

be construed as a potential conflict of interest.

Copyright © 2016 Esteban, Royo, Urarte, Zamarreño, Garcia-Mina and Moran.

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